1. Field of the Invention
The invention relates to a digital communications system comprising at least a transmitter and a receiver for transmitting a signal modulated on the basis of a first constellation, said receiver, which is intended to receive samples of said signal and make decisions with respect to said samples, further including a timing recovery device of the synchronous type which includes a phase-locked loop adjusted by means of an adjustment error.
The invention also relates to a receiver for digital modulations, intended to receive samples of a signal modulated on the basis of a first constellation and make decisions with respect to said samples, which includes a timing recovery device of the synchronous type which comprises a phase-locked loop adjusted by means of an adjustment error.
The invention finds highly significant applications in the field of synchronous communications. The clocks of the transmitter and receiver are in fact to be synchronized to optimize the sampling instants of the received signal.
2. Description of Related Art
Let us assume that a(nt) are the symbols transmitted by the transmitter at the rate T, x(nT+.tau.) is the signal sampled by the receiver with an offset of .tau., and h is the total transfer function of the transmission channel. The relation between transmitted symbols and received symbols can be expressed in the following manner: ##EQU1## In addition to the desired data a(nT)h(.tau.), there appears an intersymbol interference term which is a function of the offset .tau.. The error resulting therefrom in the received symbol will be used for correcting the offset .tau..
The article by Mueller and Muller entitled "Timing Recovery in Digital Synchronous Data Receivers", published in IEEE Transactions on Communications, Vol. 24, No. 5, May 1976, recommends to adjust the phase-locked loop by a function f derived from the channel response h and which symmetrizes the response at steady-state, that is to say, for .tau. verifying f(.tau.)=0. In practice, the channel response h is not available and thus an estimate of this function is used which itself is derived from samples received by the receiver. The estimate used is, for example, given by the following expression: EQU .epsilon..sub.n =Ree*.sub.n-1 (d.sub.n -d.sub.n-2)!
where
Re() indicates the real part of the complex number in brackets, PA1 d.sub.n is the decision made on the n.sup.th received sample x.sub.n, PA1 e.sub.n =x.sub.n -d.sub.n is the corresponding error for sample x.sub.n, PA1 and e.sub.n * is the conjugate of e.sub.n. PA1 said first error, when said first error is lower than a given threshold, PA1 and, if not, said second error. PA1 comparing means for comparing the coordinates of the received samples with the boundaries of said constellation, PA1 synchronization detection means, PA1 storage means for storing coordinates of said adjustment error, PA1 computing means for computing said adjustment error based upon the stored coordinates, PA1 check means for checking said storage means for storing a coordinate of said adjustment error only if the corresponding coordinate of the received sample is situated outside the boundaries of the first constellation, when said receiver is not synchronized.
This estimate in effect leads to symmetrizing the channel response at steady-state, since EQU Reh(T+.tau.)-h(-T+.tau.)!=0
This estimate .epsilon..sub.n is a function of the error e.sub.n-1 caused in sample x.sub.n-1. However, its use as an adjustment error of the phase-locked loop of the timing recovery device of the receiver for modulations in a large number of states yields insufficient results. In effect, whereas the distance between two points of the constellation used becomes smaller when the number of states of the constellation increases, the probability of having a false decision and thus a false error estimate becomes greater.
It is an object of the invention to overcome this disadvantage.